This invention relates to vacuum fluorescent displays in general and more particularly, to a structure wherein a control electrode or grid is formed substantially coplanar with the anode and wherein the voltage required by the control electrode in order to completely cut off the tube from display is compatible with conventional MOS drive circuitry.
Vacuum fluorescent display tubes for displaying numerals or other characters have been widely used in computers, measuring apparatus, calculators and the like in order to provide a visual display of information. In conventional vacuum fluorescent displays, a cathode, grid and anode are formed within an evacuated envelope, at least one side of which is transparent. Typically a plurality of segmented anode patterns are formed on an insulating base which is mounted within the vacuum envelope. The anode segments are typically formed in the pattern of a figure "8" so that the numerals "0" through "9" may easily be displayed. Each of the anode segments is covered with a fluorescent material such as various phosphors. The cathode is positioned between the anode and the transparent side of the vacuum envelope. The cathode, however, does not interfere with visual observation of indicated characters and is generally formed of a very fine, almost invisible, wire capable of providing electronic emission when heated. Typically, a grid formed of a plurality of very fine wires is positioned between the cathode and anode. A separate grid is generally provided for each pattern of segmented anodes such that by applying a proper bias voltage to the grid it is possible to prevent any of the anode segments from luminescing even though a voltage may be applied between the anode and cathode. A typical vacuum fluorescent display is described in more detail in U.S. Pat. No. 3,508,101, patented Apr. 21, 1970.
A vacuum fluorescent display such as above described has enjoyed considerable success in the electronics industry. In an attempt to further reduce the costs associated with manufacturing such a display, however, it would be desirable to form the grid on the insulating base that supports the anode segments, rather than form the grid as a suspended electrode such as described in the aforementioned patent. If the control grid could be formed coplanar with the anode segments, less expensive manufacturing techniques could be utilized in fabricating the display. For example, the grid material could be screened onto the insulating base. Further, the display could be made more compact, and since the grid would not be positioned between the person viewing the display and anode segments, the display would be brighter.
One proposal for forming a grid coplanar with the anode segments is described in U.S. Pat. No. 3,668,466 granted June 6, 1972. Applicant has discovered, however, that while the technique described in this patent would provide a measure of improved manufacturing capability, the structure is not suitable for present day displays. For example, if the structure includes only one grid which surrounds the anode segments and is coplanar therewith, it is not possible to prevent anode segments directly underlying the cathode from luminescing unless a relatively large voltage, on the order of 50 volts, is applied to the grid. This is undesirable, since such a structure cannot be controlled using conventional metal oxide semiconductor circuitry; that is, conventional MOS circuitry such as used in calculators, central processing units, etc. have a reverse bias breakdown voltage on the order of 35-40 volts. If larger voltages than this are required, it is necessary to use bipolar interface circuitry between the display and MOS circuit or a special configuration of MOS transistors such as described in U.S. Pat. No. 3,818,245 in order to prevent reverse bias junction breakdown of the MOS transistors. The difficulties in using MOS circuitry to switch large voltages are described in more detail in the aforementioned U.S. Pat. No. 3,818,245.